1. Field of the Invention
The present invention relates to a light emitting display device, and more particularly, to a light emitting display device which is capable of reducing hysteresis of a driving transistor to improve picture quality, and a method for driving the same.
2. Discussion of the Related Art
Recently, various flat panel display devices which are small in volume and weight compared with a cathode ray tube have been developed, and a light emitting display device using a light emitting element which has a high luminous efficiency, excellent brightness, wide viewing angle and high response speed, among the flat panel display devices, has been especially highlighted.
The light emitting element has a structure where a light emitting layer, which is a thin film emitting light, is disposed between a cathode electrode and an anode electrode, and a characteristic where excitons are generated in the light emitting layer by injecting electrons and holes into the light emitting layer and recombining them therein and light is emitted from the light emitting layer when the generated excitons falls to their low energy states. Such light emitting elements are classified into an inorganic light emitting element and an organic light emitting element according to materials of the light emitting layer.
FIG. 1 is a circuit diagram of a pixel cell of a general light emitting display device.
Referring to FIG. 1, the pixel cell of the general light emitting display device, denoted by reference numeral 10, includes a pixel circuit 12 and a light emitting element 14 formed in an area defined by a data line DLm, a scan line SLn and a driving voltage line PL.
A data voltage is supplied to the data line DLm, and a scan signal is supplied to the scan line SLn. Also, a driving voltage of a constant level is supplied to the driving voltage line PL.
The pixel circuit 12 includes a switching element ST, a driving transistor DT, and a capacitor Cst. Here, the switching element ST and driving transistor DT are p-channel (or P-type) metal oxide semiconductor (PMOS) transistors.
The switching element ST supplies the data voltage from the data line DLm to a first node N1 in response to the scan signal supplied to the scan line SLn.
The driving transistor DT supplies current corresponding to the data voltage supplied to the first node N1 to the light emitting element 14 using the driving voltage supplied to the driving voltage line PL.
The capacitor Cst stores a voltage corresponding to the data voltage supplied to the first node N1, and then holds an ON state of the driving transistor DT for a period of one frame when the switching element ST is turned off.
The light emitting element 14 emits light by the current corresponding to the data voltage, supplied from the driving voltage line PL via the driving transistor DT. At this time, current I flowing to the light emitting element 14 can be expressed by the following equation 1:I=β/2(Vgs−Vth)2=β/2(Vdata−Vdd−Vth)2  [Equation 1]
In the equation 1, I represents the current flowing to the light emitting element 14, Vgs represents a gate-source voltage of the driving transistor DT, Vth represents a threshold voltage of the driving transistor DT, Vdata represents the data voltage, and β represents a constant.
In the general light emitting display device as mentioned above, the current as in the equation 1 is supplied to the light emitting element 14 by the pixel circuit 12 to turn on the light emitting element 14 so as to display an image.
However, in this general light emitting display device, a negative data voltage is always applied to the gate electrode of the driving transistor DT through the switching element ST, so that the gate-source voltage of the driving transistor ST is always negative. As a result, the hysteresis of the driving transistor DT increases as shown in FIG. 2, resulting in a problem that the current corresponding to the data voltage cannot be supplied to the light emitting element 14 as it is.
In detail, a first curve C1 can be obtained by measuring source-drain current Ids of the driving transistor DT while varying the gate voltage of the driving transistor DT having hysteresis from a low voltage to a high voltage. Also, a second curve C2 can be obtained by measuring the source-drain current Ids of the driving transistor DT while varying the gate voltage of the driving transistor DT having hysteresis from a high voltage to a low voltage. As a result, the general light emitting display device is problematic in that the threshold voltage Vth of the driving transistor DT is subject to a variation ΔVth due to the hysteresis of the driving transistor DT.
FIG. 3A shows a display state of an image of a chess pattern in the general light emitting display device, and FIG. 3B shows a display state of an image of the same gray scale pattern in the general light emitting display device immediately after the image of the chess pattern is displayed in the display device.
A degradation in picture quality due to the hysteresis of the driving transistor DT will hereinafter be described with reference to FIGS. 3A and 3B in association with FIG. 2.
When the image of the chess pattern shown in FIG. 3A is displayed in the light emitting display device, white areas A and black areas B are displayed on a display panel of the light emitting display device. At this time, a white data voltage is applied to the gate electrode of each driving transistor DT formed in each white area A, and a black data voltage is applied to the gate electrode of each driving transistor DT formed in each black area B.
When the image of the same gray scale pattern is displayed on the display panel of the light emitting display device immediately after the image of the chess pattern is displayed on the display panel, it is ideal that a gray scale image of the same brightness is supposed to be displayed on the entire screen of the display panel.
However, in the case where the driving transistor DT of the light emitting display device has hysteresis, each driving transistor DT formed in each white area A and each driving transistor DT formed in each black area B have different threshold voltages Vth, thereby causing each light emitting element 14 in each white area A and each light emitting element 14 in each black area B to display different brightnesses. That is, as shown in FIG. 3B, when the image of the same gray scale pattern is displayed on the display panel immediately after the image of the chess pattern is displayed on the display panel, brightness C of a gray scale pattern displayed in each white area A is displayed more darkly than brightness D of a gray scale pattern displayed in each black area B.
In conclusion, the general light emitting display device has a disadvantage in that picture quality is degraded due to an afterimage formed as the image of the same gray scale is displayed with a different brightness value because of an increase in the hysteresis of the driving transistor.